Lubricating oils used in passenger cars and heavy duty diesel engines have changed over the years. Today's engines are designed to run hotter and harder than in the past. However, an adverse affect of runner hotter is that oxidation of the oils increases as the operating temperature of the oil increases. Oxidation of the oils may lead to a viscosity increase in the oil and the formation of high temperature deposits caused by agglomerated oxidation by-products baking onto lubricated surfaces. Accordingly, certain phosphorus and sulfur additives have been used to reduce engine oil oxidation.
However, the next generation of passenger car motor oil and heavy duty diesel engine oil categories may require the presence of lower levels of phosphorus and sulfur containing antioxidant additives in the formulations in order to reduce contamination of more stringent pollution control devices. It is well known that sulfur and phosphorus containing additives may poison or otherwise reduce the effectiveness of pollution control devices.
With regard to the above, a need exists for a lubricating additive that provides excellent antioxidant properties and is more compatible with pollution control devices used for automotive and diesel engines. Such additives may contain phosphorus and sulfur or may be substantially devoid of phosphorus and sulfur.